how to pass data (tuples) to worker processes?

Hi,

I'm learning how to use the "background worker processes" commited in
9.3. The usage basics are quite nicely illustrated in the worker_spi
extension (kudos to those who designed the feature / extension).

I'm not quite sure how to pass data between the regular backend and a
worker. Implementing the channel (socket/pipe/...) itself is not a big
deal, that's IPC 101, but deciding which data to copy (and how) is.

Say I need to forward a tuple to the worker process - e.g. from a
nodeAgg node, so that the worker can build the hash table. Is there
something (a rule of a thumb, method, ...) that would help me to
identify the pieces of data that need to be copied?

Or do I need to do the go through the objects and decide what to copy
and how on my own?

regards
Tomas